1. Field of the Invention
This invention is related to a pilot structure, a method, and a receiver for multi-carrier cellular communications defining a specific Time Division Multiplexer/Multiplexing pilot pattern and pilot sequence for a sub-frame with short system information.
2. Description of the Related Art
Wireless communication systems are widely deployed to provide various types of communication such as voice, packet data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users sequentially or simultaneously by sharing the available system resources. Examples of such multiple-access systems include Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, and Orthogonal Frequency Division Multiple Access (OFDMA) systems, other types of multi-carrier access schemes, or any combinations of these.
An accurate estimate of the response of a wireless channel between a transmitter and a receiver is needed in order to effectively transmit data on the available sub-bands. Channel estimation is typically performed by sending a pilot from the transmitter and measuring the pilot at the receiver. Since the pilot is made up of symbols that are known a priori by the receiver, the channel response can be estimated as the relation of the received pilot symbols over the transmitted pilot symbols. This relation may include any channel dependent information as amplitude, phase, frequency shift, angular spread, interference, noise, etc.
Pilot transmission represents overhead in a wireless communication system. Thus, it is desirable to minimize pilot transmission to the extent possible. However, because of noise, fading, Doppler, interference, angular dispersion and other artifacts in the wireless channel, a sufficient amount of pilot energy needs to be transmitted frequently enough in order for the receiver to obtain a reasonably accurate estimate of the time-variant channel response. Because the contributions of physical scatterers and the propagation paths to the channel response vary over time, the pilot transmission needs to be repeated regularly. The time duration over which the wireless channel may be assumed to be relatively constant is often referred to as a channel coherence time. The repeated pilot transmissions need to be spaced significantly closer in time than the channel coherence time to maintain high system performance. Similarly, for a wideband transmission, the pilot spacing in frequency also has to be sufficiently tight, to be able to estimate the full frequency dependent channel, which possibly extends over the coherence band. The coherence time of a channel may depend, for instance, on the velocity of the receiver. The coherence bandwidth of a channel may depend, for instance, on the delay spread of the channel.
In the downlink of a wireless communication system, a single pilot transmission from an access point (or a network element or a base station or a base station controller) may be used by a number of terminals to estimate the response of the distinct channels from the access point to each of the terminals. Furthermore, the pilot signals of different access points need to be separable form each other, from random data and from noise or interference to allow reliable estimation of the channel between the access point and the terminal. In the uplink, the channel from each of the terminals to the access point typically needs to be estimated through separate pilot transmissions from each of the terminals.
A special Short System Information (SSI) message present in every frame provides information pertaining to the wireless communication system, which is receivable by all terminals. Thus, the SSI has to be received by the terminals with good probability in all propagation and mobility conditions, in any reasonable cell deployment, and with any of the specified system bandwidths. In addition to frame synchronization, system bandwidth in a scalable bandwidth system (as Evolved Universal Terrestrial Radio Access Network (E-UTRA)), and the operation bandwidth is determined by receiving the SSI during the initial synchronization. In handover, the system bandwidth is given in the neighbor list, and decoding of the SSI is actually necessary mainly for frame timing detection. There is, therefore, a need in the art for a pilot structure and method that enables faster and more reliable finding of the system information message during the initial synchronization and during handover (preparation) in all propagation, mobility and interference conditions. The mobility conditions may include a receiver velocity up to 350 km/h.